Valves for use with vacuum equipment are essentially different from valves used in conventional systems whose objective is to control the flow of a liquid or gas. In such conventional systems, some leakage of the liquid or gas can usually be tolerated. In the case of vacuum valves, however, even the slightest leak involves not merely the loss of a liquid or gas, but total destruction of the vacuum in the vacuum equipment controlled thereby. In addition, vacuum destroying gases originate not only through leaks in the valves, but also from the materials used in the construction of conventional valves. For that reason, it is most desirable to use completely sealed, all-metal valves in conjunction with vacuum equipment. Specifically, it is not feasible to use organic materials for the seals of such valves.
All-metal valves have been produced in the prior art for use with vacuum equipment, and such valves usually comprise a metal gate which is moved at right-angles to a pair of aligned openings in the valve housing which are to be opened and closed thereby. Such gate valves usually include appropriate means for causing the gate to be sealed against a seat surrounding one of the valve openings when the gate is moved to its closed position.
Copending Application Ser. No. 842,110 provides an improved construction for such a valve in which mechanical linkages and mechanisms required to actuate the gate are minimized, and in which the valve gate is actuated and forced against a seat around the opening in the valve housing by a simple mechanism which requires a minimum of moving parts. The valve housing is preferably hermetically sealed and evacuated so as to reduce the pressure differential across the gate seal to a minimum. This serves to obviate any tendency for molecular leaks to occur across the seal.
An all-metal seal is used in the valve assembly of the Copending Application Ser. No. 842,110, and the assembly is formed entirely of metal, so that it may be free from organic materials which could destroy the vacuum within the equipment controlled by the valve. Moreover, the valve assembly of the last-named copending application may operate in wide extremes of ambient temperatures without destroying the seals between its gate and the corresponding seat. Moreover, the valve assembly of the last-mentioned copending application can be placed in areas where high levels of radiation occur which would otherwise damage organic seals, and the like, and cause such seals to give off gases or vapors.
The valve assembly of the present invention represents a modification which can be incorporated into the valve assembly described in the last-named application, or in other gate valve assemblies, which enables the assembly to provide a smooth bore with an electrically conductive surface when the valve is open, which effectively short-circuits any stray radio frequency currents produced by the aforementioned radiation, which currents otherwise could create a hazard.
The requirement fulfilled by the valve of the present invention arises wherever advanced high energy physics experiments such as storage rings, linear accelerators, ion sources and similar devices are used. In such situations, isolation valves with the special feature of the valve of the present invention is required. That is, the valve, when open, must present a relatively smooth bore throughout its opening, with no large pockets being exposed through which radio frequency energy can flow, and to provide an electrical short-circuit to radio frequency energy from seat-to-seat of the valve.
The storage ring referred to above is a closed loop containing a beam which may be composed of a variety of subatomic particles, that is, protons, electrons, positrons, and the like. It is essential that the pressure inside the storage ring be very low, that is, in the ultra high vacuum range. Since the storage ring if of considerable volume, it requires vast pumping capacity and operation time to lower its internal pressure. Should a leak develop, the entire storage ring would lose vacuum. To protect against such a catastrophic loss, it is necessary to break up the ring into a series of sectors, and then to connect the sectors by means of valves, such as the valves of the present invention. The valves serve the dual purpose of providing continuity to the ring as well as selective isolation of one or more sectors which might develop leaks.
The linear accelerators referred to above have a similar use for valves such as the valves of the present invention. The difference between a linear accelerator and a storage ring is that the high energy beam, instead of traveling in a continuous circular path, is a straight beam with a source at one end and a target at the other.
When conventional valves are used, an undesirable feature from the standpoint of beam integrity arises. This is due to the difference in the dimension of the structure carrying the beam, which is essentially a tube of uniform cross-section, and the internal dimensions of the valve body. The beam depends on the internal surface of its enclosure for the maintenance of concentration and coherence. As the beam passes through the sector, an "image" current passes simultaneously along the sector wall. When the beam arrives at any given valve, the "image" current encounters an abrupt non-uniformity in its path through the valve housing when usual prior art valves are used. This disruption between the uniformity of the beam and the "image" current path causes the sharpness, coherence and intensity of the beam to be reduced, until eventually there is total disintegration of the beam. This undesirable result is obviated by the valve of the present invention which provides a continuous path for the beam through the valve.
Specifically, the prior art valve in its open position presents cavities in relation to the sector. Such cavities could support resonant modes which provide feedback and cause beam instability. It is desirable, therefore, to maintain a uniform enclosure for the beam at all points in the storage ring. The effect of the valve cavity must be reduced and this is done by means of the short-circuiting mechanism of the present invention. Without the short-circuiting mechanism, the image current would have to travel through the much longer and irregular path of the prior art valve body, thereby causing the beam to lose coherence and intensity as mentioned above.